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After starting up my machine in verbose mode (cmd + v at start-up) I want to view the generated log messages, to debug an error I'm receiving (the error). The error is only displayed on the screen briefly during startup, before the machine reboots. As a result, there's no time to read it properly.

Does OS X provide a separate log file for start-up messages, or are these buried within the kernel.log and system.log files? If they are buried within these files, is there a quick way to jump to the point where the system boots, within the console viewer?

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+1 for a good question! –  daviesgeek Sep 21 '11 at 21:25
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Starting in at least Snow Leopard and possibly Leopard, all logging on the system goes through the Apple System Log (ASL) mechanism, which is like a central daemon and database for all log messages. The ASL daemon also makes sure that appropriate log messages that meet certain traditional criteria are written out to traditional flat text log files like system.log and kernel.log, but everything goes to the ASL database as well.

For more information, see the man pages for syslog(1), logger(1), syslog(3), asl(3), syslog.conf(5), asl.conf(5), syslogd(8), etc.

You can use syslog(1) to issue queries against the ASL database. Also, /Utilities/Console.app provides a GUI for generating and saving custom queries for the ASL database.

I'm pretty sure that the kernel's message buffer (traditionally viewed with dmesg(8)) gets dumped into the ASL database as soon as boot has proceeded far enough for syslogd to be running, so you shouldn't need to call dmesg directly, except through force of habit. Or if your system isn't booting far enough for syslogd to be running.

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Does this work for previous boots? The problem I was experiencing when I wrote this question (resolved now, thankfully) was only occurring intermittently. So say, every other boot, it was Kernel panic on me, and what I wanted to do was on the next successful boot, review the log file, find the previous boot, and try and debug. –  purpletonic Mar 28 '12 at 16:08
    
@purpletonic Yes, everything that gets sent to ASL gets put in a database, where it stays for a default of 7 days (this can be adjusted with asl.conf(5)). So if you were debugging booting problems and rebooted several times the same day, they'd be there by default. And if it was a longer-term problem, you could adjust ASL's retention period to keep stuff longer. –  Spiff Mar 28 '12 at 16:57
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Open Terminal.app and run

sudo dmesg

(you'll be asked for your password)

This will display system messages from the last startup until now (you'll find the startup messages at the top of the output). You may try

sudo dmesg | more

this will output those messages page by page.

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I myself wondering how to get the log most appropriate to the startup without access to the console/terminal. It crashed at startup but you can get access to the file system through the install disk (I had a dual linux boot available). The file is:

/var/log/kernel.log

If you can access the console within the operating system, it's better to follow the other suggestions and use the appropriate commands.

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